1. Field of the Invention
This invention relates to a servo positioning apparatus used, for example, to position a read/write head of a magnetic recording disk, and more particularly, it relates to efficient tuning of a filter used in a feedback loop of a servo control circuit.
2. Description of the Related Art
Servo positioning apparatuses have been widely employed for quickly and accurately positioning a mechanical element, such as a magnetic head of a magnetic recording disk apparatus. A typical servo control system for a recording disk apparatus is illustrated in FIG. 1. Magnetic recording disks 10 are mounted on an axle 11 driven by a spindle motor 12 thus enabling the magnetic recording disks 10 to spin. Magnetic heads 13 are mounted on accessors 140 which are connected to a movable part of a voice coil motor 14. The accessors 140 are moved in an essentially radial direction relative to the magnetic recording disks 10 by a coil 141 and a magnet 142 of the voice coil motor 14. Subsequent usage of the terms position and speed will refer to movement of the accessors 140 in the radial direction relative to the magnetic recording disks 10. This is known in the art as the seek direction.
The disks 10, axle 11, motors 12, 14 and heads 13 are typically installed in a magnetic disk unit 1 enclosed in a disk enclosure 16. The magnetic disk unit 1 is mounted in the main chassis of a magnetic disk apparatus but is detachable therefrom, for servicing, etc.
A coarse controller (i.e., a velocity control circuit) 2 generates a velocity error signal .DELTA.V based upon a position signal PS obtained from a servo signal generated by the magnetic head 13. The details of the coarse controller 2 are described below. The position signal PS indicates the radial position of the magnetic head relative to the adjacent track and is generally designed to be zero when the head is at the center of a track being traced.
A fine controller or position controller 3 generates a position error signal .DELTA.P based upon the position signal PS. The details of the fine controller 3 are described below. A movement controller 4 composed of a microprocessor calculates the number of tracks needed to be jumped according to an instruction from a host processor unit (not shown). The movement controller 4 switches between the coarse and fine modes by outputting a mode switching signal MS to a switch 5 and outputting an instruction to both the coarse controller 2 and the fine controller 3, each to drive the voice coil motor 14. The coarse controller 2, the fine controller 3, the movement controller 4, and the switch 5 compose a servo controller CT.
When the coarse mode is selected by the movement controller 4, the mode switching signal MS causes switch 5 to connect to terminal "a". As a result, a driving current I.sub.m proportional to the velocity error signal .DELTA.V is supplied from the power amplifier 6 to the voice coil motor 14 via switch 5. Thus, the magnetic head 13 is moved to a destination track by a predetermined velocity schedule depending on the difference between a target velocity V.sub.c and an actual velocity V.sub.r of the magnetic head 13 obtained from the position signal PS. As illustrated in FIG. 2, upon detecting the magnetic head 13 having reached the destination track according to the position signal PS, the movement controller 4 issues a mode switching signal causing switch 5 to connect to terminal "b", thus selecting the fine controller 3. In FIG. 2, .alpha. is the width of a track to be traced, and MS is the mode switching signal for switch 5.
The fine controller 3 delivers the position error signal .DELTA.P to the power amplifier 6 via switch 5 for controlling the positioning of the magnetic head within the destination track. Accordingly, a driving current I.sub.m proportional to the position error signal .DELTA.P is supplied from the power amplifier 6 to the voice coil motor 14.
Thus, a driving current I.sub.m proportional to the position error signal .DELTA.P or the velocity error signal .DELTA.V is supplied *from the power amplifier 6 to the voice coil motor 14. As a result, the magnetic head 13 is controlled to trace or move to a destination track.
Moreover, as shown in FIG. 3, the moving parts of the servo positioning apparatus naturally have a mechanical resonance. Therefore, a notch filter 32 (FIG. 4) has been widely employed in the servo loop (i.e., feedback loop) to dampen an undesirable oscillation of the heads 13 caused by the mechanical resonance.
The details of the fine controller 3 are shown in FIGS. 4, 5(a) and 5(b). A fine position amplifier 30 generates, from the position signal PS, a signal which is zero volts when the head is located at the center of a track, and otherwise has a positive or negative voltage proportional to the deviation from the center of the track to one side or the other. A clamp circuit 31 generates a clamped position signal by modifying the signal output from the fine position amplifier 30 to have saturations at predetermined positive and negative levels. The clamped position signal output from the clamp circuit 31 is then passed through a notch filter 32 and a high frequency filter 33 to a phase compensation circuit 34. The notch filter 32 has an attenuation characteristic shown in FIG. 5(b), such that the attenuation is maximum at the mechanical resonant frequency f.sub.0. The high frequency filter 33 attenuates the high frequency gain of the servo loop to obtain a stable servo operation. The phase compensation circuit 34 also helps provide a stable servo operation, where an integrator circuit 34a integrates the output from the high frequency filter 33 for integral control (I); an amplifier 34b amplifies the output from the high frequency filter 33 for proportional control (P); and a differentiator circuit 34c differentiates the output from the high frequency filter 33 for derivative control (D). An adder circuit 34d adds the outputs from the integrator circuit 34a, the amplifier 34b and the differentiator circuit 34c, then outputs the position error signal .DELTA.P to the switch 5.
The notch filter 32 is typically composed of a twin T-type filter as illustrated in FIG. 5(a). A twin T-type filter is configured as follows. A parallel connection of capacitors C.sub.1 and C.sub.2 connected to a connecting point of resistors r.sub.1 and r.sub.2 forms a first T-type network. A series connection of resistors r.sub.3 and r.sub.4 connected to a connecting point of capacitors C.sub.3 and C.sub.4 forms a second T-type network. By connecting these two T-type networks in parallel, the twin T-type filter is formed. In addition, operational amplifiers may be added to the twin T-type filter according to the following conditions. The output from resistor r.sub.2 and capacitor C.sub.4 is input to an amplifier AMP1 having a unity gain. The output of amplifier AMP1 is then divided by resistors r.sub.5 and r.sub.6, and the divided output voltage appearing across r.sub.6 is input to another amplifier AMP2 having a unity gain. Output of the second amplifier AMP2 is fed to a return terminal t.sub.3, which is the connecting point of capacitors C.sub.1, C.sub.2 and resistor r.sub.4.
A notch frequency f.sub.0 of the twin T-type filter is given by the following formula, where R indicates a resistance value R equal to the resistance of each resistor r.sub.1 through r.sub.4, and C indicates a capacitance value C equal to the capacitance of each capacitor C.sub.1 through C.sub.4. ##EQU1## where, R=r.sub.1 =r.sub.2 =r.sub.3 =r.sub.4
C=C.sub.1 =C.sub.2 =C.sub.3 =C.sub.4
Accordingly, the notch frequency f.sub.0 can be adjusted by varying the resistance value R and/or the capacitance value C.
A problem, however, with the twin T-type filter is that each of the four resistors r.sub.1 through r.sub.4 or each of the four capacitors C.sub.1 through C.sub.4 must be replaced according to formula (1) for adjusting the resonant frequency while keeping the attenuation constant, when the disk unit 1 is replaced for servicing, etc.
Another conventional notch filter 32 previously used in a servo control circuit is a bridged T-type filter illustrated in FIG. 6. This type of filter has two major problems. First, the available attenuation, such as -12 dB, is not adequate and two, three or four capacitors or resistors must be replaced to adjust the resonant frequency while keeping the attenuation constant.
Each disk unit 1 does not always have the same resonant frequency even when the products are of the same design. Furthermore, if the design is changed for an improvement, etc., the resonant frequency consequently changes. The notch filter is typically mounted on a printed circuit board, which is installed in a main chassis on which the disk apparatus is installed. The exchange of the three or four circuit elements is a time consuming job requiring a precise instrument. Moreover, exchange of the printed circuit board is of course expensive. In any event, the cost for the production of a notch filter, as well as its servicing, is considerable. In order to avoid this problem, the notch filter may be mounted within the replaceable disk unit 1, in which the notch frequency is tuned at the factory, as disclosed in Japanese Unexamined Patent Publication Tokukai Sho No. 58-188374. However, the problem still remains because the frequency adjustment is a troublesome job even in a production line of a factory.